مجله پژوهش فیزیک ایران
سال یازدهم شماره 2 (تابستان 1390)

An analytical model for two-pass optical amplifier with arbitrary beams overlap has been developed which generalized the classical theory of Frantz-Nodvik for single pass amplifier. The effect of counterpropagating beams on gain and output energy fluence included in the model. Moreover, the appropriate limiting relations for two special cases of weak input signal and saturation state of the amplifier gain have been derived. The results indicate that for complete beams overlap, the gain and output energy have the least values. The model predictions are consistent with experimental observations and exact analytical model for two-pass amplifier when beam propagation paths are coincided.

Recently, a new carbon modification, namly n-diamond, have been reported, whose structure is still a matter of debate. It is important to note that the synthesis of n-diamond was carried out in the presence of hydrogen or methan. In this work we evaluate the structural stability of five fcc-CHx phases by means of first-principle calculation. The total energy is obtained as a function of the isotropic, tetragonal and rhombohedral deformations for the bulk structures. First, we analyze the C2H (cuprite), CH (zincblende), CH (rocksalt) and CH2 (fluorite) structures.It is found that the four systems show a minimum in the total energy for the isotropic and rhombohedral deformations, but are unstable against tetragonal deformation. In the second part, we explore the structural stability of CH2 in the pyrite structure. We find that CH2 (pyrite) with the hydrogen atoms defined by the internal parameter u=0.35 and a lattice parameter of 3.766 Å is elastically stable, providing a possible explanation for the experimental observation of fcc-carbon in materials prepared in the presence of hydrogen or methan. In final, we calculate density of states, band structure and EELS spectrum of CH2 (pyrite) and compare them with n-diamond.

A three-body model is devised to study differential and total cross sections for the excitation of helium atom under impact of energetic protons. The actual process is a four body one but in the present model the process is simplified into a three-body one. In this model, an electron of helium atom is assumed to be inactive and only one electron of the atom is active. Therefore, the active electron is assumed to be in an atomic state with a potential of the nucleus, T, being screened by the inactive electron, e, and, thus, an effective charge of Ze. As a result, the ground state, 11S, or the excited states, 21S and 21P, wave function of the active electron is deduced from similar hydrogenic wave functions assuming effective charge, Ze for the combined nucleus (T+e). In this three-body model, the Faddeev-Watson-Lovelace formalism for excitation channel is used to calculate the transition amplitude. In the first order approximation, electronic and nuclear interaction is assumed in the collision to be A(1)e= and A(1)n=, respectively. Here, A(1), Txy, |i> and |f> are the first order transition amplitude, the transition matrix for the interaction between particles x and y, the initial state and the final state, respectively. The transition matrix for the first order electronic interaction implemented into A(1)e is approximated as the corresponding two-body interaction, Vxy. In order to calculate first order nuclear amplitude A(1)n, the near-the-shell form of transition matrix TPT is used. Calculations are performed in the energy range of 50 keV up to 1MeV. The results are then compared with those of theoretical and experimental works in the literature.

In this research, we have investigated the effect of spin orientation of quarks on the production of the Bc(1S) heavy mesons. We have calculated the fragmentation functions at initial scale µ°. Then we have modified to µ these fragmentation functions by using Altrali-Parisi equation. Also, we have calculated the transverse momentum distributions of the differential cross sections. Our results are in agreement with the experimental data for Tevatron Run II.

The thermal and mechanical properties of pure cobalt were studied by using the molecular dynamics (MD) simulation technique, in the temperature range from 200 K up to melting point. The Cleri-Rosato many-body potential was used as interatomic potential. This MD simulation was employed at the constant pressure, constant temperature (NPT) ensemble. The temperature and pressure of the system were controlled by Nose-Hoover thermostat and Berendsen Barostat, respectively. We computed the variation of the cohesive energy, the order parameter, the bulk modulus, and the elastic stiffness constants at different temperatures for this metal. The thermal expansion coefficient and the isobaric heat capacity were calculated as a function of temperature by fitting the lattice parameter and the cohesive energy to a quadratic polynomial equation, respectively. Our computed results showed a good agreement with the experimental results available.

In the present paper, we calculate the effect of disorder on the density of states and localization of eigenstates of zigzag and armchair nano-ribbons. Both Anderson disorder model and quantom percolation are considered. The eigenstates of the disordered hamiltonian is calculated in a tight–binding approximation and the localization of eigenstates are characterized using the inverse participation ratio (IPR). Based on IPR data, the transport properties in the presence of disorder is discussed.

Co nanowires were fabricated onto the nanoporous aluminum oxide arrays using ac electrodeposition technique. The influence of frequency and pH on the magnetic properties and crystal structure of symmetric (18-18 V) and non-symmetric (18-12 V) electrodeposited Co nanowires was investigated. A direct relation was seen between saturation magnetization, squareness and coercivity of the samples prepared with various deposition frequencies and electrolyte acidities. Coercivity of Co nanowire arrays prepared by non-symmetric anodization voltage changed between 600 to 700 Oe while it was varied from 1280 to 1630 Oe for those fabricated by symmetric deposition voltages. The easy axis was perpendicular and parallel to the wires axis in the Co nanowires arrays prepared by non-symmetric and symmetric electrodeposition voltage, respectively.

Non-commutative field theory as a theory including the Lorentz violation can be constructed in two different ways. In the first method, the non-commutative fields are the same as the ordinary ones while the gauge group is restricted to U(n). For example, the symmetry group of standard model in non-commutative space is U(3)×(2)×U(1) which can be reduced to SU(3)×SU(2)×U(1) by two appropriate spontaneous symmetry breaking. In contrast, in the second method, the non-commutative gauge theory can be constructed for SU(n) gauge group via Seiberg- Witten map. In this work, we want to find the relation between the NC-parameter and the Lorentz violation parameters for the first method and compare our results with what is already found in the second one. At the end, we obtain new limits on non-commutative parameter by using the existing bounds on the Lorentz Violation parameters.

Wind is the carrier of pollutants and any other gaseous or particle matters in the atmosphere. Stable atmosphere with low wind provides favourable conditions for high contamination of pollutants in urban areas. The importance of mesoscale atmospheric flows in air pollution dispersion has been recognized in the past three decades and has been the focus of intensive research; both observational and numerical. Mesoscale or local scale circulations are more prominent when the synoptic pressure gradients are weak, allowing horizontal temperature contrasts to develop, which in turn lead to mesoscale pressure perturbations. Tehran, a city which is situated at the southern foothills of the Alborz Mountain chain has an average elevation of 1500m, and covers an area of 864 km2. Alborz Mountains have a significant influence on the dynamics and thermodynamic modification of wind regime over the city. At the same time, the Urban Heat Island effect (UHI) can cause its own mesoscale flow, complicating an already complex local scale flow. The topography and the urban fabric can cause slope flows, mountain flows, and valley flows amongst many other factors. Th is paper focuses on the use of state-of-the-art atmospheric numerical model – The Weather Research and Forecasting (WRF) – in an ideal situation to study the characteristics of the mesoscale flow systems that prevail over Tehran when air quality is unfavourable. So average sound of Radiosonde at Mehrabad station, for almost all the fair days of cold seasons from 2005 to 2008 was selected as an ideal initial condition and boundary condition with 10 × 10 km spatial and 12-hour temporal resolution. The simulations were carried out for a 3-day period in December 2005 when an aircraft, due to low visibility caused by high concentration of air pollution, crashed 2 miles away from the end of runway into an inhabited area. Three simulations were prepared. For the first experiment, called control run, we used the default urban setting of Tehran. In the second simulation, urban properties of Tehran were removed completely from the land-use fed to the model to investigate the effect of urban area on thermally induced circulation in Tehran. This simulation was called NoURB simulation. To investigate the role of the roughness in the urban area, a 3rd simulation was prepared. In this simulation, which was referred to as HiFric simulation, three urban categories were used; class 31 of USGS land use/land cover was used for “Low Intensity Residential”, which included areas with a mixture of constructed materials and vegetation. These areas were most commonly included as single-family housing units in which the population densities were lower than those in high intensity residential areas. Class 32 of USGS represented “High Intensity Residential” which included highly developed areas where people resided in large numbers. Finally class 33 of USGS was used for “Commercial/Industrial/Transportation”, which included infrastructures (e.g. roads, railroads, etc.) and all highly developed areas not classified as High Intensity Residential. The results indicated that urban areas near complex topography can increase transfer of material (pollution) and energy from boundary layer to the free atmosphere. Besides the ideal simulation, we investigated some observational aspects of the transition time by wind persistence charts. The results showed that we have a 2 to 3-hour lag time at the evening transition of northern and southern part of the city which could improve the pollution potential in this period. This fact may play an important role in Tehran megacity management.

Determination of eye absorbed dose during head & neck radiotherapy is essential to estimate the risk of cataract. Dose measurements were made in 20 head & neck cancer patients undergoing 60Co radiotherapy using LiF(MCP) thermoluminescent dosimeters. Head & neck cancer radiotherapy was delivered by fields using SAD & SSD techniques. For each patient, 3 TLD chips were placed on each eye. Head & neck dose was about 700-6000 cGy in 8-28 equal fractions. The range of eye dose is estimated to be (3.49-639.1 mGy) with a mean of maximum dose (98.114 mGy), which is about 3 % of head & neck dose. Maximum eye dose was observed for distsnces of about 3 cm from edge of the field to eye.

Topological analysis of the electronic charge density is introduced as a new tool for studying the electronic properties of the materials. In this method, the eigen values of the Hessian matrix of the electronic charge density as an scalar field are used to estimate the strength of the atomic bonds. We employ this method to study the half-metallic phase transition of MnAs in zinc blende structure. The results show that the topology of the electron density is preserved in this phase transition. So the total magnetization as the order parameter of the system changes continually and the phase transition is regarded as a second order transition. The geometrical changes observed in the electron density are interpreted by investigating the electronic structure.

The ultracold atoms fermion gas such as 6Li undergo superfluidity state. The transport quantities of these fluids have a direct dependence on the transition probabilities. Here, by obtaining possible processes in p-wave superfluid, we have shown that only binary processes are dominate at low temperatures.